<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
   <style>
       *{
           margin: 0;
           padding: 0;
       }
       div{
           width: 500px;
           height: 4000px;
           background: #000;
           margin: 0 auto;
       }
       .as{
           width: 100px;
           height: 100px;
           background: red;
           /*相对于浏览器的当前窗口，脱离文档流*/
           position: fixed;
           right: 0;
           bottom: 100px;
       }
       .df{
           width: 100px;
           height: 100px;
           background-color: hotpink;
       }
   </style>
</head>
<body>
    <div></div>
    <div class="as"></div>
    <div class="df"></div>
</body>
</html>